For more than 30 years, unbonded post-tensioned cables have been used in concrete structures such as parking garages, apartment and office buildings, etc. Generally, steel cables of high tensile strength which are encased in plastic or impregnated paper sheaths are used. The concrete structure which is to be subjected to a tensile load, is formed with hollow sheaths or ducts, positioned in the direction of the highest expected tensile load. Enclosed in the sheath is a post-tensioning cable. The cable and sheath combinations are placed in the concrete forms prior to filling thereof and, once the concrete has attained a specific compressive strength, the cables are tensioned by way of anchors placed at either end thereof. Hydraulic jacks are used to stress or elongate the cables and the cables are locked in their anchors by wedges before the hydraulic jacks are removed. The tension in the steel cables imparts a prestress force to the concrete and gives it the capacity to support its own weight and the anticipated load to be applied to the structure. Thus, when the concrete structure is later put into use under tensile load conditions, any tensile load applied in the direction of the prestressing will, up to the degree of applied prestress, serve merely to relieve the compressive stresses upon the concrete structure while not placing the structure under actual tension
Because the tensile load bearing capacity of the concrete structure depends almost exclusively on the tensile stress carrying capabilities of the cables, corrosion of the cables, which is usually caused by water that infiltrated the cable sheath, must be prevented to guarantee the integrity of the structure. In order to maintain the reliability of the cables during the anticipated service life of the structure, each cable is provided with a protective film of grease applied during manufacture. Nevertheless, since the early 1980's reports of corroded cables have been accumulating. Numerous incidents have been reported where a post-tensioned cable was corroded to the point where it would break. However, once such a highly tensioned cable breaks, the sudden release of the energy stored in the cable can result in the cable shooting out the edge of a concrete slab or bursting through the top or bottom thereof which could lead to serious injury of a person standing within reach of the failed cable. Thus, post-tensioned concrete structures with corroded cables potentially pose great danger to human life and a means is desired which will greatly reduce and preferably stop the corrosion of an installed post-tensioned cable. There also exists the need for a means which will provide a sufficiently large bond between an installed post-tensioned cable and the surrounding concrete structure to prevent a sudden release of the stored energy should the cable break. Methods and materials used for the grouting of post-tensioned cables are disclosed in the following patents:
U.S. Pat. No. 5,079,879 to Rodriguez; PA1 U.S. Pat. No. 5,138,808 to Bengtson et al; PA1 Canadian Patent No. 1,005,465 to Schupack; and PA1 Canadian Patent No. 1,225,253 to Harris et al.
Rodriguez (U.S. 5,079,879) discloses the use of a foam filler for the sealing of a tubular anchor plate extension which surrounds a tendon at the anchor plate. However, the foam is placed prior to the pouring of the concrete and only around that portion of the cable which is inside the extension and not in the void between the cable and its sheath. The problem of corrosion of post-tensioned cables in existing concrete structures is not addressed.
Bengtson et al (U.S. 5,138,808) teach a wall structure of hollow, loosely stacked masonry blocks which are held together by post-tensioning rods and are filled with a polyurethane foam for increased heat insulation. Again, neither the corrosion problems encountered with post-tensioned cables in existing concrete structures nor any solutions thereto are disclosed. The polyurethane foam is not used for grouting purposes, but only functions as insulating material. The significant differences in bonding and setting properties between hydrophillic and hydrophobic polyurethane resins are not disclosed.
Schupack (CA 1,005,465) teaches the grouting of post-tensioned cables with a concrete mixture. He acknowledges that excess water in the concrete grouting mixture poses a big problem, since it can result in a lack of bonding or sealing and, subsequently, lead to corrosion of the cable. However, in the sealing of existing non-grouted post-stressed tendons which have been in service for some years and possibly subject to water infiltration, it is hard, if not impossible, to predict the amount of water accumulated within the cable sheath. Furthermore, many generally horizontally extending tendons are not level but have rising and declining portions so that infiltrated water accumulates in pockets at the low points of the cable sheath. It will be readily appreciated by the skilled artisan that these pockets of water seriously impede the effectiveness of a cement grout. The use of non-cementous grouting material is not discussed.
Harris et al (CA 1,225,253) disclose the use of a hydrophillic epoxy resin for the grouting of post-tensioned cables. However, differing amounts of water present in the cable sheath will affect the curing and bonding properties of the resin composition. The resin will not fully cure, if insufficient water is present in the sheath. On the other hand, if more water is present in the sheath than is required for the resin to fully cure, some residual water will remain which can lead to additional corrosion of the cable. The use of grouting compositions other than hydrophillic epoxy resin is not disclosed.
Although these patents provide some guidance to the art skilled person in solving the corrosion problems encountered with post-tensioned cables in existing concrete structures, the problems caused by water infiltrated into the cable sheath still remain. These problems are corrosion due to residual water left after grouting and incomplete bonding between the grout and the cable if water is enclosed in the resin or if insufficient water is present in the cable sheath for a hydrophillic grouting resin to fully cure. Therefore, a grouting system for post-tensioned cables in existing concrete structures is desired which will overcome these problems.